Extend discovered checks regardless of SEE

miguel-l · snicolet · commit 9050eac59564 · 2019-02-01T16:27:21.000+01:00
A simple idea, but it makes sense: in current master the search is extended for checks that are considered somewhat safe, and for for this we use the static exchange evaluation which only considers the `to_sq` of a move. This is not reliable for discovered checks, where another piece is giving the check and is arguably a more dangerous type of check. Thus, if the check is a discovered check, the result of SEE is not relevant and can be ignored. STC: LLR: 2.96 (-2.94,2.94) [0.50,4.50] Total: 29370 W: 6583 L: 6274 D: 16513 http://tests.stockfishchess.org/tests/view/5c5062950ebc593af5d4d9b5 LTC: LLR: 2.95 (-2.94,2.94) [0.00,3.50] Total: 227341 W: 37972 L: 37165 D: 152204 http://tests.stockfishchess.org/tests/view/5c5094fb0ebc593af5d4dc2c Bench: 3611854
diff --git a/src/search.cpp b/src/search.cpp
@@ -953,11 +953,13 @@ namespace {
           else if (cutNode && singularBeta > beta)
               return beta;
       }
-      else if (    givesCheck // Check extension (~2 Elo)
-               &&  pos.see_ge(move))
+
+      // Check extension (~2 Elo)
+      else if (    givesCheck
+               && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
           extension = ONE_PLY;
 
-      // Extension if castling
+      // Castling extension
       else if (type_of(move) == CASTLING)
           extension = ONE_PLY;
 
